The present invention relates to the field of chemical vapor deposition with the assistance of a plasma, and more specifically, it relates to an apparatus for plasma activated chemical vapor deposition, the apparatus comprising, disposed one after another:
a plurality of transformation means for transforming the state of respective precursors of a plurality of deposition materials to cause each of them to pass from an initial state to the gaseous state;
a plurality of charging means for charging vector gases with respective gaseous precursors, each precursor-charged vector gas constituting a predetermined gaseous mixture;
a plurality of transfer means for transferring said predetermined gaseous mixtures to a plasma reactor having microwave excitation comprising a reaction enclosure, a microwave generator, and at least one waveguide interposed between said generator and the enclosure and providing non-resonant coupling; and
injection means for injecting said predetermined gaseous mixtures into the reaction enclosure.
Apparatuses of the type to which the invention applies are designed to form deposits of special materials, in particular of ceramics, on substrates. It is thus possible, in particular, to make thermal barriers which, when deposited on an appropriate protective coating (e.g. of MCrAlY material), enable turbine blades to be made that are well protected thermally: the operating temperature of the metal portions remains below the allowed limit for the base material and the protective coating, while the temperature of the gas at the inlet of the turbine can be considerably hotter (50.degree. C. to 100.degree. C.), thereby correspondingly increasing the efficiency of the turbine.
At present, three methods are known for obtaining thermal barriers:
1. The most widely used technique is plasma spraying. Grains of partially-stabilized zirconia powder are inserted into a plasma jet; they melt therein and are accelerated so as to be projected at high speed against a facing substrate. They solidify thereon rapidly and they adhere thereto by mechanically engaging roughnesses previously formed on the surface of the part, generally by sandblasting. The resulting coatings have a flaky structure that results from the stacking up of droplets that have flattened and solidified in lens-shapes, with solidification being accompanied by microcracking. The highest performance deposits are constituted by a layer of zirconia partially stabilized by 6% to 8% by weight of yttria deposited on an underlayer of MCrAlY alloy (where M.dbd.Ni and/or Co and/or Fe), itself deposited by plasma spraying under a controlled atmosphere. Generally, the ceramic layers obtained in this way are about 300 .mu.m thick. This technique leads to deposits having microcracks and that may be constituted by metastable phases, with deposition speeds being very high, of the order of 100 .mu.m/min. It should nevertheless be observed that the method is directional and that parts of complex shapes are difficult or even impossible to cover. Furthermore, the roughness of such deposits makes finishing treatment necessary to achieve a surface state that is aerodynamically satisfactory.
2. The method of evaporation under electron bombardment makes use of an electron beam emitted by a heated filament. The beam is accelerated by application of an electric field and it is directed by means of a magnetic field onto the material to be evaporated, in the present case a bar of yttrium-containing zirconia. Under the effect of such electron bombardment, the species are evaporated and condensed on the substrate that is placed facing the source. The substrate is optionally biased and is preheated and/or heated during the deposition operation. The results obtained by implementing this method present certain advantages:
the roughness of the layer obtained in this way is better adapted to aerodynamic flow;
the column structure of the deposit improves its thermomechanical behavior;
it has higher resistance to erosion; and
the layer adheres better.
However, it should be observed that making a coating of yttrium-containing zirconia at a high deposition speed (100 .mu.m/h) requires high electrical power to evaporate the bar of refractory oxide. In addition, implementation of this technique requires considerable investment and a large amount of know-how. Furthermore, this method is likewise directional, i.e. parts that are complex can be difficult or even impossible to coat.
3. The radiofrequency cathode sputtering method makes it possible to deposit thin layers of yttrium-containing zirconia at low deposition speeds of the order of 1 .mu.m/h. In this method, a material raised to a negative potential is subjected to bombardment by positive ions. The atoms of the material are ejected in all directions and condensed, in particular on the substrate placed facing it so as to have a deposit formed thereon. Systems have been used to make deposits of insulating materials (deposits of oxides in particular) with possible modifications (magnetron, spraying in a reactive atmosphere, etc.) in order to increase deposition speeds (up to a few .mu.m/h). With this technique, it is certainly easier to control the composition of deposits than it is with the method of evaporation under electron bombardment; however deposition speeds are much slower and, as in the two preceding methods, deposition takes place directionally.
An essential object of the invention is to provide a novel deposition apparatus which enables the respective advantages of chemical vapor deposition and of plasma assistance to be combined, and in particular: a method which is non-directional, takes place at a lower temperature and at an increased deposition speed, and a deposit which has a structure that is controlled.